From the prior art, bead-type cation exchangers of the polymethacrylic acid type are already known. This is a class of cation exchangers which can be used in numerous different applications in practice. An important field of use is water treatment technology, in which polyvalent cations, such as, for example, calcium, magnesium, lead or copper, but also carbonate anions, can be removed. A particular advantage of the methacrylate-based cation exchangers is that their regeneration can be performed not only using strong acid, such as hydrochloric acid or sulphuric acid, but also using carbon dioxide. A further field of application of cation exchangers of the polymethacrylic acid type is purification and separation of biologically active components, e.g. antibiotics, enzymes, peptides and nucleic acids, from their solutions, for example from reaction mixtures and from fermentation broths.
For the said applications, not only gel-type but also macroporous cation exchangers of the polymethacrylic acid type are required. The terms macroporous and gel-type are described in detail in the specialist literature, for example in Seidl, Malinsky, Dusek, Heitz, adv. Polymer Sci., Vol. 5 pages 113 to 213 (1967).
Cation exchangers of the polymethacrylic acid type can be produced by what is termed inverse suspension polymerization. In this process, an aqueous solution of methacrylic acid and crosslinker is dispersed in an organic solvent, such as cyclohexane, for example, as continuous phase to give spherical droplets, and these droplets are cured by polymerization at elevated temperature using a free-radical initiator.
Solvent-free production processes which are to be preferred, inter alia for ecological reasons, are also already known. In this case methacrylic acid itself is not used, but an alkyl ester of methacrylic acid in combination with a crosslinker is converted into bead-type crosslinked polyalkyl methacrylate by suspension polymerization in water which is converted into the polymethacrylic acid cation exchanger by downstream saponification. U.S. Pat. No. 2,340,111 describes, for example, insoluble copolymers having carboxyl groups for removing cations from water, with the carboxyl-containing copolymers being able to be produced by saponifying acrylic ester copolymers. DE 960 858 describes a process for producing gel-type polymers usable as cation exchangers by suspension polymerization of esters of acrylic acid and/or methacrylic acid and by subsequent saponification of the resultant polymers.
Whereas saponification of polyacrylic alkyl ester polymers to give polyacrylic acid polymers may be carried out very readily, the analogous reaction using methacrylate polymers, because of the higher stability to hydrolysis of the methacrylic esters, requires drastic reaction conditions, e.g. temperatures above 150° C. These drastic conditions are undesirable in practice, since they require increased technical resources and the resultant reaction product can change in its structure as a consequence of the temperature stress.
The object of the present invention is to provide a simple process for the production of gel-type and macroporous cation exchangers of the polymethacrylic acid type.